Ban entered the Nazi death camp through the infamous 'Arbeit Macht Frei' (Work Makes You Free) gate to see exhibitions that document the inhuman conditions suffered by camp inmates.
Accompanied by a former Auschwitz detainee Marian Turski, he walked through the red brick barracks, which still house huge mounds of shaved hair, shoes, baggage, and spectacles that were taken from inmates upon their arrival.
"Nothing can truly prepare one for this epicenter of evil," Ban said, "where systematic murder unique in human history reached its atrocious climax."
The UN secretary-general laid flowers at the executions wall, where thousands of inmates, mostly Polish resistance members, were shot to death.
Ban also visited the Chevra Lomdei Mishnayot synagogue, the only building still standing that testifies to the vibrant pre-war Jewish life that once existed in the adjacent town of Oswiecim.
At the concentration camp's gate, Banmet the former Israeli chief rabbi and current chairman of Yad Vashem Yisrael Meir Lau, who survived the Holocaust as a child.
"The world must never forget, deny or downplay the Holocaust," said Ban. "We will stand in eternal solidarity with the survivors and safeguard their testimony so their legacy will never die."
Between 1940-1945, some 1.1 million people, mostly Jews, died in gas chambers or from starvation, disease and forced labor in the Auschwitz-Birkenau death camp that Nazi Germany built in occupied Poland. It was one of the biggest of the Nazi camps, has become a synonym for the Holocaust.
In an address made at the camp, Ban expressed concern that many nations are still plagued by ethnic and religious persecution. "Even today, the fire smolders. Anti-Semitism retains its hold in too many places. In Europe and elsewhere, migrants, Muslims, Roma and other minorities face rising discrimination - and find too few defenders," he said. "We will continue to shine a light on these unspeakable crimes so that they [may] never be repeated."
"Auschwitz-Birkenau," added Ban, "is not simply a register of atrocities. It is also a repository of courage and hope.
Ban also visited Krakow, which is located 50 kilometers (31 miles) from Auschwitz. On Tuesday he will join a UN climate conference in Warsaw.
Boutros Boutros-Ghali was the first UN secretary-general to visit Auschwitz in 1995.
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